The invention relates generally to the field of communications and, more particularly, to the use of echo cancellation and double-talk detection in communications units.
In a hands-free communication unit, received audio is conveyed to a user, or a group of co-located users, through a speaker system. This allows a user to listen to voice transmissions from a transmitting station without requiring the user to wear a headset or hold a telephone earpiece in order to hear transmissions from a far-end transmitter. Additionally, a hands-free communications unit allows a group of co-located users to participate in a conversation with another user located at a far-end transmitter in a natural manner.
When a transmission from a far-end transmitter is received and conveyed to a user through a speaker, a portion of the audio from the speaker is typically coupled into the microphone of the hands-free communications unit at the receiving end. This is primarily due to the fact that the speaker and microphone are usually placed within close proximity of each other. Thus, the hands-free communications unit must discriminate between audio signals coupled into the microphone which result from speaker echo as well as voice signals from the user or group of users.
When a user is talking while a signal is being received from a far-end transmitter, this condition is referred to as double-talk. Typically, a hands-free communications unit employs some technique of determining if the user is talking, and if a signal from a far-end transmitter is being received while the user is talking. Current double-talk detectors make use of attenuators in order to reduce the audio output of the speaker which, in turn, reduces the level of the echo coupled into the microphone of the hands-free communications unit. However, this attenuation produces an unnatural condition where the users of the hands-free communications unit hear an intermittently attenuated version of the signal from the far-end transmitter.
Other techniques employ the use of a linear model in order to estimate the audio level of the signal from the far-end transmitter in order to subtract the echo from the outgoing audio signal. Typically, these techniques then compare the power of the outgoing signal to the power of the received signal in an attempt to determine if the outgoing signal contains speech in a potential double-talk condition. In addition to being highly dependent on the performance of a recursive filter, these techniques often rely on fixed thresholds and an implicit time dependency in the relationship between the outgoing and incoming signals. These factors limit the accuracy of current techniques. Alternatively, methods of monitoring deviations of the recursive filter coefficients which indicate divergence due to an outgoing signal can be used. However, such techniques are computationally expensive and highly dependent on the effectiveness of the recursive filter.
Therefore, it is highly desirable for the hands-free communications unit to employ an echo cancellation mechanism with improved double-talk accuracy which requires minimal processing resources and allows for changes in the timing of the echoed signal as it is radiated from the speaker and coupled into the microphone. Such a system would result in a lower cost hands-free communications unit which provides the capability to engage in a natural conversation between users of the hands-free communications unit and a far-end transmitter.